Structural geometry and strain distribution within eastern Umtanum fold ridge, south-central Washington

Umtanum Ridge is one of the best-exposed Yakima ridges formed by folded basalt flows in south-central Washington. An analysis was made of the structural geometry and strain distribution in the deformed basalt layers exposed on the ridge at Priest Rapids Dam. The purpose of the analysis was to gain an understanding of the distribution and orientations of the small-scale structures (faults, breccias, joints) around the anticlinal structure. From this we can assess the relative strain intensity and distribution around the fold, and use this information, along with the mapped profile shape of the fold and associated faults, to construct a balanced section leading to constraints on the tectonic models of the Columbia Plateau. The strain distributions and structural geometries within Umtanum Ridge accord well with an asymmetric kink-fold geometry with predominantly flexural strains in the steep limb; however, the internal cataclastic flow is not penetrative at field-observation scale. Discrete flexural slip has occurred, both within and along flow contacts, as well as some internal shatter brecciation and faulting between and across the flow-parallel faults. The Umtanum fault, a large reverse fault, is associated with the anticline and, on the basis of the reconstructed section, is conjectured to have formed out of the kink-like fold at depth. Slickenside striae orientations on faults developed during folding are generally perpendicular to the fold axis. This is interpreted to mean that the dominant movement of basalt layering during folding was perpendicular to the fold axis. The mechanical continuity between the anticline and the adjacent syncline to the north is interpreted to have not been disrupted until late in the fold history. Because of this hypothesized continuity and because the dominant relative movement direction of displacement was perpendicular to the fold axis, movement on the Umtanum fault is intepreted to have been predominantly dip-slip. It is further inferred that if any regional strike-slip component was present in the Pasco Basin, it does not manifest itself obviously in the Umtanum fold kinematics.